1. Field of the Invention
The present invention relates to high pressure discharge lamps using ceramic discharge tubes and processes for the production thereof.
2. Related Art Technique
In the above high pressure discharge lamp, plugging members (ordinarily called ceramic plugs) are inserted into both end portions of the ceramic discharge tube to close these end portions, a through-hole is provided in each of the plugging members, and a metallic current conductor having a given electrode system fixed thereto is inserted into the through-hole. An ionizable light-emitting material is sealed in an inner space of the ceramic discharge tube. As such a high pressure discharge lamp, a high pressure sodium light-emitting lamp, a metal halide lamp, etc. are known. In particular, the metal halide lamp has a good color rendering property. Use of ceramics as materials for discharge tubes has made it possible to use such high pressure discharge lamps at high temperatures.
In such a discharge lamp, it is necessary to effect gas-tight sealing between the end portions of the ceramic discharge tube and respective electrode unit-holding members. A main portion of the ceramic discharge tube takes a tubular shape or a barrel shape of which both end portions are reduced in size or a straight cylindrical shape. The ceramic discharge tube is made of, for example, a sintered alumina body. In order to seal the end portions of the ceramic discharge tube, for example, JP-A-6 318435 discloses the following structure. That is, plugging members are inserted into interiors of end portions of the ceramic discharge tube, and held there. A through-hole is formed in each of the plugging members in an axial direction thereof, and a slender electrode unit-holding member is fixedly inserted into the through-hole. The plugging member is made of a cermet containing both alumina and a metal constituting the electrode unit-holding member at such a given ratio that the coefficient of thermal expansion of the plugging member may fall between the coefficient of thermal expansion of the electrode unit-holding member and that of the ceramic discharge tube.
In the formation of the above sealed structure, it is designed that the inner diameter of each end portion of the ceramic discharge tube becomes slightly smaller than the outer diameter of the plugging member if the ceramic discharge tube is fired in such a state that a preform of the plugging member is not inserted into a preform of the ceramic discharge tube. Consequently, the plugging member is firmly radially inwardly tightened and held inside the end portion of the ceramic discharge tube. This is the same as to the plugging member and the electrode unit-holding member.
However, the present inventors have further advanced investigations upon such sealed structures, and discovered that they had the following problems. That is, the plugging member and the electrode unit-holding member are sealed based on a pressure between them. However, since the discharge lamp repeatedly undergoes a number of cycles between turning on and turning off, it is necessary that reliability of the sealed portion is further enhanced judging from the difference in thermal expansion. In particular, in case of the metal halide having high corrosive property, a sealed structure having high corrosion resistance and high reliability needs to be developed.